This invention relates to an aural transmitter device in which the amplification level of the voice amplifier is controlled according to the input level of a sound signal applied through an aural transmitter (such as a microphone) or the like, whereby the effect of environmental noise is minimized.
In general, a voice amplifier for amplifying sound signals outputs a sound signal in proportion to the input level thereof.
When the output sound signal of an aural transmitter is amplified, the voice for the announcer is applied, as a relatively high sound pressure to the aural transmitter, and therefore during aural transmission environmental noises may be ignored, as they are masked by the voice.
However, when aural transmission is suspended, the environmental noises only are amplified, and are therefore offensive to the ear. Especially when the device is installed on a vehicle, the environmental noise is offensive to the ear because low frequency noises such as engine noises are emphasized.
In order to eliminate this difficulty, a method has been proposed in the art, as disclosed in the Handbook "600-Type Telephone System," pp. 26-27, published July 1964 by the Telecommunications Association, in which the voice amplifier is operated with a low gain when the voice input is equal to or lower than a predetermined level Pa and with a high gain when the same is higher than the predetermined level Pa. However, this method is disadvantageous in that, when the voice input level is around Pa, the amplified output level varies abruptly and thus the aural transmission does not seem natural. Further, if the difference between the two gains is reduced in order to eliminate the aforementioned drawback, it is rather difficult to eliminate the drawback attributable to environmental noise.